Method of electroplating aluminum or its alloy with porous hard chromium



3,090,609 METHOD OF ELECTRUPLATEWG ALUMINUM OR ITS ALLQY WITH POROUS HARD CHROMIUM Kirniyoshi Katayose, 49 Nishidaimachi Shimura, Itabashilm, Tokyo, Japan N Drawing. Filed- Sept. 11, 1961, Ser. No. 137,051 1 Claim. (61. 204-43) This invention relates to the method of electroplating aluminum or its alloys with porous hard chromium, and especially relates to the method of electroplating the internal surfaces of cylinders of internal combustion engine made of aluminum or its alloys with porous hard chromium to form an exfoliation and abrasion resisting surface on which lubricating oil spreads well.

The electroplating of aluminum or its alloys with porous hard chromium has previously been considered to be very difficult for the following reasons: the existence of oxidized film on and pin holes in the surface of the base metal; the erosiveness of the base metal; and the difference of the expansion characteristics of the base and applied metals. In recent years, because of rapidly increasing demands for light engines, there have been many attempts to improve the performance of such engines by electroplating the internal surfaces of the cylinders of engines made of aluminum or its alloys with hard chromium, taking advantage of its resistance to abrasion, erosion and friction. Prior to the present invention, no acceptably satisfactory results have been achieved in connection with the problem here under consideration. Particularly, it has been ditficult thus far to obtain a durable and firmly adherent film by electroplating with porous hard chromium that will permit lubricating oil to spread well in addition to the above characteristic peculiar to the hard chromium.

The principal object of this invention is to solve the above mentioned problems and successfully electroplate aluminum or its alloys, particularly in connection with the internal surfaces of engine cylinders made of these metals, with durable and firmly adherent porous hard chromium. This objective has been achieved as the result of many studies on the pretreatment and operation of electroplating.

It is among the objects of this invention to provide an electroplating method comprising removing the oxidized film from the surface of the base metal and making the surface rough by acid cleaning with a mixed solution of hydrochloric acid and hydrofluoric acid, then producing the porous oxidized film by anodic oxidation in a solution of chromic acid or other acid, such as sulfuric acid or phosphoric acid, and then electroplating in a chromic acid bath with ordinary soft chromium, and further, electroplating in a chromic acid bath with hard chromium, and finally electroplating with porous hard chromium by means of a chromic acid bath.

The effects of the above method may be explained as follows: -In the first place, the oxidized film on the metal surface can be remove-d by the action of hydrochloric acid and hydrofluoric acid, since aluminum or its alloys are cleaned by the mixed solution of hydrochloric acid and hydrofluoric acid, and at the same time, the metal surface can be made rough. In the second place, although an oxidized film may be produced on the metal surface due to anodic oxidation in the solution of chromic acid, it will serve to increase the depth of the holes produced as result of the process. Consequently, the ladherence of the plating film increases, since the electroplating is effected through these holes, and the plating film will adhere to the metal surface more effectively than in conventional pretreatment methods that employ anodic oxidation only. In the third place, since ordinary soft chromium is electroplated in a chromic acid bath, the above efiect will occur, and it is also possible to perform the electroplating operation comparatively easily and produce a firm bond. In the fourth place, sufficient adhesion on the same metal can be obtained since hard chromium is electroplated on a soft chromium plating film, and since the base surface of aluminum or its alloy is coated with the plating film of soft chromium, the electroplating of hard chromium which has been considered comparatively difficult can be performed readily to produce a firm bond regardless of the nature of the base metal. In the fifth place, since porous hard chromium is electroplated on the hard chromium plating film and, as in the above process, the plating is performed on the same metal, sufiicient adhesion is ensured, and it is also possible to electroplate with porous hard chromium irrespective of the base material since the two chromium plating layers, soft and hard chromium layers, exist already. Thus, according to the invention, the abovementioned two processes are especially employed, and moreover, the three plating layers of electroplated soft, hard and porous hard chromium are produced in turn so that perfect adhesion may be performed and the resistance to abrasion, erosion and heat will be acquired. As a result, porous hard chromium of good quality on which lubricating oil will spread well can be plated on the internal surfaces of engine cylinders by employing this method.

A preferred form of this invention is set forth as follows.

(l) Pretreatment:

A. Acid cleaning- Composition of solution Hydrochloric acid percent 15 Hydrofluoric acid do 5 Soaking time min 5 Temperature Normal temperature B. Anodic oxidation- Composition of bath-Chromic acid g/l 100 Current density (anode) A/dm. 50-70 Temperature 50 Treatment time min 3-10 (2) Electroplating operation:

A. Electroplating with ordinary soft chromium- Composition of bath Chromic acid g/l 400 Sulfuric acid g/l 4 Voltage v 4.5-5 Current density (cathode) A./dm. 35-45 Temperature C 25-30 Electroplating time hr 1 B. Electroplating with hard chromium- Composition of bath Chromic acid g/l 200 Sulfuric acid g/l 2 Voltage v 3 .8-4 Current density (cathode) A./dm. 50-60 Temperature C 30-55 Electroplating time hr 1 C. Electroplating with porous hard chromium- Composition of bath Chromic acid g/l 200 Sulfuric acid g/l 2 Voltage v 4-4.5 Current density (cathode) A/dm. -100 Temperature C 50-55 Electroplating time hr 1 What I claim is: A method of electroplating articles of aluminum and its alloys with porous hard chromium comprising cleaning said articles by the mixed solution of hydrochloric acid and hydrofluoric acid, then treating by means of anodic oxidation in a solution containing approximately 100 grams per liter of chromic acid While maintaining the temperature of the solution at approximately 50 C. and While maintaining the anodic current density in the range of from 50 to 70 amperes per square decirneter for from 3 to minutes, and then electroplating with ordinary soft chromium in 1a chromic acid bath containing sulfuric acid and approximately 400 grams per liter of chromic acid while maintaining the sulfuric acid within a range of a ratio of chromic acid to sulfuric acid of approximately 100 to 1, land while maintaining the temperature of the bath within the range of to C. and while maintaining the current density (cathode) in the range of to amperes per square decimeter and the voltage in the range of 4.5 to 5 volts for 1 hour, and next, electroplating with 'hard chromium in a chromic acid bath containing sulfuric acid and approximately 200 grams per liter of chromic acid while maintaining the sulfuric acid within a range of a ratio of chromic acid to sulfuric acid of approximately 100 to 1, and while maintaining the temperature of the bath within the range of 30 to C. and while maintaining the current density (cathode) in the range of 50 to amperes per square decimeter and the voltage in the range of 3.8 to 4 volts for 1 hour, and lastly, electroplating with porous hard chromium in a chromic acid bath containing sulfuric acid and approximately 200 grams per liter of chromic acid while maintaining the sulfuric acid Within a range of a ratio of chromic acid to sulfuric acid of approximately -100 to 1, land while maintaining the temperature of the bath within the range of 50 to 55 C. and While maintaining the current density (cathode) in the range of to amperes per square decimeter and the voltage in the range of 4 to 4.5 volts for 1 hour.

References Cited in the file of this patent UNITED STATES PATENTS 1,988,645 Bowden Jan. 22, 1935 2,678,908 Tucker May 18, 1954 FOREIGN PATENTS 568,161 Great Britain Oct. 4, 1944 763,336 Great Britain Dec. 12, 1956 

